Friction materials, in particular, friction materials used in vehicle powertrains, are often formed from a base material, e.g., fibrous paper, which is saturated with a resin, e.g., phenolic resin, and subsequently cured. The cured friction material is more durable than the base material on its own, as the resin acts as a glue-like material holding the fibrous paper together. The combination can be quite strong. For example, such a friction material may be used in a vehicle transmission where it will be exposed to aggressive environmental conditions such as elevated temperatures typically ranging from 180° C. to 200° C. Exposure to temperatures above about 220° C. can result in decomposition of some phenolic resins.
Tung oil is added to the phenolic resin as it increases the flexibility of the cured resin, and improves durability of friction material in pressure cycles at higher temperatures. However, friction material performance decreases when tung oil is added, i.e., the low speed and static friction coefficients increase and the dynamic friction coefficient decreases which results in higher operating temperatures. The foregoing results in decreased performance in clutch applications.
Some known friction materials delaminate under aggressive conditions, e.g., shearing of layers. Additionally, some known friction materials crack or wear away.